The Universal Mobile Telecommunication Service (UMTS) standard provides a compatibility standard for cellular mobile telecommunications systems. The UMTS standard ensures that a mobile station (MS), or a user equipment (UE), operating in a UMTS system can obtain communication services when operating in a system manufactured according to the standard. To ensure compatibility, radio system parameters and data transfer procedures are specified by the standard, including protocols governing digital control messages and bearer traffic that are exchanged over an air interface.
The UMTS standards provide, in 3GPP TS 25.346 (Third Generation Partnership Project Technical Specification 25.346) v0.5.0 and 3GPP TS 23.846 v6.0.0, for a provision of a Multimedia Broadcast Multicast Service (MBMS) service by a UMTS communication system to UEs serviced by the system. The MBMS service provides for a multicast and unicast of MBMS data, typically in a format of Internet Protocol (IP) data packets to one or more of the UEs. In order to ensure that the air interface resources of the UMTS communication) system are not wasted, the system must first estimate the number of recipients, that is subscribed UEs, in a cell providing MBMS deals. Based on the estimated number of recipients, the system then determines whether to establish a Point-To-Multipoint (PTM) communication channel in the cell or a Point-To-Point (PTP) channel to each recipient. When the estimated number of recipients in the cell exceeds an operator defined threshold, the system establishes a PTM channel in the cell. When the estimated number of recipients in the cell is less than the operator defined threshold, the system establishes a PTP channel to each subscribing MS in the cell.
Typically, the system estimates the number of recipients based on a number of UEs subscribing to MBMS services that are active over established connections. Based on the estimate, a Radio Network Controller (RNC) included in a UMTS infrastructure determines whether to establish a PTM communication channel in the cell or a PTP communication channel to each UE. The RNC then broadcasts a MBMS notification via a Node B, typically a base transceiver station (BTS), and a control channel to all UEs in the cell. The notification typically includes an identifier associated with the MBMS service. In response to receiving the MBMS notification, each UE in the cell that subscribes to the MBMS service may then convey a connection request, typically a Radio Resource Control (RRC) connection establishment request, to the RNC via an access channel. Upon receiving the connection requests from each of the subscribing UEs, the RNC establishes a PTM communication channel or establishes PTP communication channels with each responding UE, whichever the RNC has determined to establish, and conveys the MBMS data to the subscribing UEs over the established channel or channels.
In order to limit the number of connection requests generated in response to the MBMS notification, it has been proposed to broadcast an access probability factor in conjunction with the MBMS notification. However, a problem arises in that, typically, the RNC is not aware of a number of idle mode UEs in a cell that have subscribed to an MBMS service. Since the idle mode UEs do not have active connections with the RNC, the RNC does not count these UEs when estimating a number of recipients of the MBMS data. When the access probability factor is set to a high value and the number of idle mode UEs subscribing to the MBMS service is also large, an access channel can be overloaded by the number of connection requests generated in response to the MBMS notification. In the event of a system overload, the UMTS standards permit an operator of the system to employ a back-off mechanism. However, back-off mechanisms produce connection set-up delays and waste RF resources as UEs must repeatedly transmit connection requests. On the other hand, when the access probability factor is set to a low value and the number of idle mode MSs subscribing to the MBMS service is small, the number of connection requests received by the RNC in response to a counting request may be insufficient to invoke establishment of a PTM channel when a PTM channel may be the most efficient scheme for disseminating the multimedia data.
Therefore, a need exists for a method and apparatus that provides a more accurate estimate of a number of mobile stations serviced by a system and subscribed to MBMS service, including idle mode mobile stations, while limiting a number of connection requests generated in response to an MBMS notification, and that provides for dynamic adjustment of an access probability factor that controls the number of connection requests.